Jet lag

Travel fatigue is general fatigue, disorientation, and headache caused by a disruption in routine, time spent in a cramped space with little chance to move around, a low-oxygen environment, and dehydration caused by dry air and limited food and drink. It does not necessarily involve the shift in circadian rhythms that cause jet lag. Travel fatigue can occur without crossing time zones, and it often disappears after one day accompanied by a night of good quality sleep.[3]

There are two separate processes related to biological timing: circadian oscillators and homeostasis.[7][8] The circadian system is located in the suprachiasmatic nucleus (SCN) in the hypothalamus of the brain. The other process is homeostatic sleep propensity, which is a function of the amount of time elapsed since the last adequate sleep episode.[8]

The human body has a master clock in the SCN and also peripheral oscillators in tissues. The SCN's role is to send signals to peripheral oscillators, which synchronise them for physiological functions. The SCN responds to light information sent from the retina. It is hypothesised that peripheral oscillators respond to internal signals such as hormones, food intake, and "nervous stimuli".[9]

The implication of independent internal clocks may explain some of the symptoms of jet lag. People who travel across several time zones can, within a few days, adapt their sleep–wake cycles with light from the environment. However, their skeletal muscles, liver, lungs, and other organs will adapt at different rates.[10] This internal biological de-synchronisation is exacerbated as the body is not in sync with the environment – a "double desynchronisation", which has implications for health and mood.[11]

Delayed sleep phase disorder is a medical disorder characterized by delayed sleeping time and a proportionately delayed waking time due to a phase delay in the endogenous biological master clock. Specific genotypes underlie this disorder. If allowed to sleep as dictated by their endogenous clock these individuals do not suffer any ill effects as a result of their phase shifted sleeping time.

North–south flights that do not cross time zones do not cause jet lag. However, crossing of the Arctic Ocean or even the North Pole (often the shortest route between northeast Europe and Alaska or the Canadian West Coast and East Asia) does cause a significant time change. Jet travel from Alaska to northeast Europe causes a pattern of jet lag very similar to an eastward flight at lower latitudes. Seasonal differences in sunlight if one crosses the equator may make a slightly disrupted sleeping pattern at the destination.

In general, adjustment to the new time zone is faster for east–west travel than for west–east. A westward adjustment takes, in days, approximately half the number of time zones crossed; for eastward travel, adjusting to the new time zone takes, in days, approximately two-thirds the number of time zones crossed.[3] Studies have shown that performance in both individual and team sports is measurably better in athletes who have flown westward to the venue than in the opposite direction.

Travelling east causes more problems than travelling west because the body clock has to be advanced, which is more difficult for the majority of humans than delaying it. Most people have an endogenous circadian rhythm that is longer than 24 hours, so lengthening a day is less troublesome than shortening it. Equally important, the necessary exposure to light to realign the body clock does not tie in with the day/night cycle at the destination.[3]

Travelling east by six to nine time zones causes the biggest problems, as it is desirable to avoid light in the mornings. Waterhouse et al.[3] recommend:

Travelling east by 10 hours or more is usually best managed by assuming it is a 14-hour westward transition and delaying the body clock.[3] A customised jet lag program can be obtained from an online jet lag calculator. These programs consider the sleep pattern of the user, the number of time zones crossed, and the direction of travel. The efficacy of these jet lag calculators has not been documented.

Travelling west causes fewer problems than travelling east, and it is usually sufficient to seek exposure to light during the day and avoid it at night.[3]

The advice is to prepare by having late sleep times the days before, and to travel daytime (not so early that early wakeup is needed) and to sleep some on board in order to be less tired at arrival and during ground transfer.

Some methods are described below.

Timed light exposure can be effective to help people match their circadian rhythms with the expected cycle at their destination; it requires strict adherence to timing.[12] Light therapy is a popular method used by professional athletes to reduce jet lag.[13] Special glasses, usually battery-driven, provide light to the eyes, thus inhibiting the production of melatonin in the brain. Timed correctly, the light may contribute to an advance or delay of the circadian phase to that which will be needed at the destination. The glasses may be used on the plane or even before users leave their departure city.[14]

Timed melatonin administration may be effective in reducing jet lag symptoms.[15] The benefit of using melatonin is likely to be greater for eastward flights than for westward ones because for most people it is easier to delay than to advance the circadian rhythm. There remain issues regarding the appropriate timing of melatonin use in addition to the legality of the substance in certain countries.[5] How effective it may actually be is also questionable.[3] For athletes, anti-doping agencies may prohibit or limit its use.[5]

Melatonin can be considered to be a darkness signal, with effects on circadian timing that are the opposite of the effects of exposure to light.[16][17] Melatonin receptors are situated on the suprachiasmatic nucleus, which is the anatomical site of the circadian clock.[18] The results of a few field studies of melatonin administration, monitoring circadian phase, have provided evidence for a correlation between the reduction of jet lag symptoms and the accelerated realignment of the circadian clock.[19]

Timing of exercise and food consumption have also been suggested as remedies, though their applicability in humans and practicality for most travellers are not certain, and no firm guidelines exist.[3][5] There is very little data supporting the use of diet to adjust to jet lag.[3] While there is data supporting the use of exercise, the intensity of exercise that may be required is significant, and possibly difficult to maintain for non-athletes.[3] These strategies may be used both before departure and after landing. Individuals may differ in their susceptibility to jet lag and in how quickly they can adjust to new sleep–wake schedules.[5]

Short-acting sleep medications can be used to improve sleep quality and timing, and stimulating substances such as caffeine can be used to promote wakefulness, though research results on their success at adapting to jet lag are inconsistent.[3]

For time changes of fewer than three hours, jet lag is unlikely to be a concern, and if travel is for short periods (three days or fewer) retaining a "home schedule" may be better for most people.[3] Sleeping on the plane is only advised if it is within the destination's normal sleep time.[3]

In the case of short duration trips, an easy way to minimize jet lag is to maintain the sleep-wake schedule from home after arriving at the destination, but this strategy is often unpractical in regard to desired social activities or work obligations.[20] Shifting your sleep schedule before departure by 1–2 hours to match the destination time zone may also shorten the duration of jet lag.[21] Symptoms can be reduced even more through a combination of artificial exposure to light and rescheduling, as it has been shown to augment phase-shifting.[22]

A short hypnotic medication has been effective to reduce insomnia related to jet lag.[23][24] In a study, zolpidem improved sleep quality and reduced awakenings for people traveling across five to nine time zones.[25] Potential adverse effects of hypnotic agents, like amnesia and confusion, should be taken into account.[26] Several cases using triazolam to promote sleep during a flight reported dramatic global amnesia.[27]